Tempering bath



Patented July 25, 1933 HORACE UNITED STATES. PATENT OFFICE NEW JERSEY C. KNERR, OF PHILADELPHIA, PENNSYLVANIA, ASSIGNOR TO AJAX ELEC- TROTHERMIC CORPORATION, 0]? AJAX. PARK, NEW JERSEY, A CORPORATION OF No Drawing. I

bath being relatively inert to the steel being treated and to the container in which the bath is contained, which will be relatively free from carbon dioxide and other substances having an undesirable effect, which .will provide a film adhering to the steel when it is withdrawn from the bath to protect it from oxidation while it is hot, and which will by convection, insure a uniform temperature within the bath, and convey heat rapidly into the article to be treated. Further desirable characteristics of this bath are that its freezing point is only slightly below the temperature of operation, so that when metal parts which have previously been preheated to a temperature in the neighborhood of 1600 Fahr. are inserted therein, a layer of the compound will freeze around the parts protecting them from too rapid heating. Furthermore, the bath does not rapidly volatilize or give off objectionable fumes or vapors or decomposeat the maximum operating temperature.

A further purpose is to manufacture a compound for use in a molten bath for'the heat treatment of high speed steels by mixing together boric acid and a compound ofbarium that will react with boric oxide,

preferably either carbonate, hydrate, or oxide, using the hydrate or oxide when it is desired to avoid the continued presence of carbonates and/or carbon dioxide gas.

Further purposes will appear in the specification and in the claims.

I- have found that the bdrates of barium make a particularly desirable material for use as a molten bath for the heat treatment of articles of high speed steel.

I find the molten material is relatively inert to steel at the operating range (2250 TEMPERING BATH Applicatitn filed July 29, 1929. Serial No. 382,073.

to 2450 Fahr.) and therefore does not seriously attack steel parts being treated or the steel pot used to contain the bath, I find that it leaves the pieces that have been immersed in it coated with a protective film after they have been Withdrawn from the bath, thereby shielding the surfaces of the treated metal from oxidation by the air.

I find further that the material at the operating temperature range (2250 to 2450 Fahr.) is so fluid as to permit active circulation of the bath by convection with a resultant desirable uniformity of temperature throughout the bath.

I find that the material has a melting point only moderately below the minimum operating temperature (2250 Fahr.) so that. it freezes around the steel parts inserted for heating until they have nearly reached the desired quenching temperature, thereby protecting them from excessively rapid or non-uniform heating.

I find that the molten bath is remarkably quiescent at the normal operating temperatures, giving off substantially no objectionable gases, and that it is remarkably permanent, itself volatilizing at a rate so slow that the material lasts a long time, in use; so that a bath, even in continuous operation, requires merely the addition of a very little new material from time to time to vmaintain the bath at its full volume.

Thematerial is relatively stable chemically, up to the maximum operating temperature, and does not decompose.

The film of salt which adheres to the tools when they are withdrawn from the bath,"

dioxide, and for this reason when the bath I is for use in heat treatment of 'these steels I prefer to avoid the presence of carbon dioxide in the material of the bath by manufacturing the material in a way that will insure its absence.

Another desirable characteristic of my material is its small change in volume during solidifying. I find that there is little or no pipe when its solidifies.

The change in volume during heating up to the melting point is also slight. This moderate solid and solid-to-liquid expansion is advantageous in that danger of bursting the container when bringing the bath up to operating temperature is reduced to a minimum. With other salts it has been necessary to remove the bath from the container while still molten in order to avoid bursting the container on reheating.

One feature of my invention is directed to a method of manufacturing the barium borate for use in my bath.

In one adaptation of my method of manu facturing I mix powdered commercial barium carbonate and powdered commercial boric acid together in proportions of about 8 to 5, by Weight, and feed the mixture in small quantities into a graphite crucible heated to a temperature of about 1400 to 1800" F ahr. L

As the materials fuse there is a substantial reduction in volume. The feeding may be continued until the crucible is nearly full of the molten compound. The bath is then heated for a period to about 2400" Fahr. to complete the reaction, after which it may be transferred to a shallow metal container, and

gredients. A quantity let cool.

The cooled solid mass is preferably then broken into suitably small pieces for use in the crucible or pot containing the bath for heat-treating the high speed steels.

When preparing the salt as above there is an evolution of gases from the reacting inof water vapor is first driven off. Carbon dioxide escapes when the materials have fused.

The evolution of carbon dioxide takes place while the bath is molten and may not be completed until the bath has been heated for some time to about 2400" Fahr. When the fusion is made in agraphite crucible the walls of the crucible to some extent reduce the carbon dioxide to monoxide.-

At high temperatures boron trioxide may be given off in small quantities.

The carbon dioxide is some of it perhaps remaining even after the reaction is apparently complete and for this reason I may in some cases prefer to avoid its presence altogether by drate or barium oxide instead of the barium carbonate.

Boric anhydride (boron trioxide) may be employed in place of the boric acid, thus eliminating the evolution of water vapor.

I have much reason to suppose that borates of calcium and strontium will act simiwhat I claim as new and desire to eliminated slowly,

using barium hy- Le1ae4e larly to those of barium when used in heating baths.

In order to preheat tools before insertion in the high temperature bath which operates at 2250" to 2450" F ahr., it is desirable to have a bath which is liquid below the preheating temperature, 1500" to 1600" Fahr. This bath must not harmfully contaminate the high temperature bath when carried over into it as an adhering film on preheated work. It may therefore preferably be a modification of the high temperature bath.

I have found that the addition of ap proximately 20% to 35% of barium chloride to the high temperature. bath above described will lower the melting point of the latter to about 1400" Fahr. The preheating bath so formed is relatively inert to the steel tools and to the container at preheating temperatures and its viscosity is such as to permit active circulation by convection at those temperatures, contributing to uniformity of temperature within the bath. Furthermore, when the Work is inserted, a thick layer of the material freezes around it, preventing excessively rapid heating, and when the properly preheated Work is removed for transfer to the high temperature bath, a thin layer adheres, thick enough to protect the surface of the work during trans fer, "but not so thick as to carry an excessive amount of the preheating bath over into the high temperature bath.

I have found that the slight addition of barium chloride to the high temperature bath during its normal life, due to the layer of preheating mixture adhering to the tools, does not cause serious contamination nor give the high temperature bath undesirable characteristics.

In view of my invention and disclosure variations and modifications to meet individual whim or particular need will doubtless become evident to others-skilledin the art, to obtain all or part of the benefits of my invention without copying the structure shown, and I, therefore, claim all such in so far 'as they fall within the reasonable spirit and scope of my invention.

Having thus described my invention, secure by Letters Patent is: v I

1. A material for use molten in a bath for heat-treating metals comprising barium borate substantially free from carbon dioxlde.

2. A material for use molten as a bath for heat-treating metals comprising a fused mixture of a barium compound containing barium oxide with boric oxide in proportions to give barium borate.

3. A material for use molten as a bath for heat-treating metals made from an approximate 8 to 5 mixture of barium carbon- 5. A material for use molten in a bath for preheating metals preliminary to heattreating in a bath containing barium borate,

comprising barium borate and barium chloride.

HORACE C. KNERR. 

